BenchDuffsDeviceMemoryCopy.cs

BenchDuffsDeviceMemoryCopy.cs

// Twitter discussion https://twitter.com/nietras1/status/1469740763760181250
// Benchmark of DuffsDevice mem copy vs plain Unsafe.CopyBlock
// |         Method | BufferSize |       Mean |     Error |    StdDev |
// |--------------- |----------- |-----------:|----------:|----------:|
// |     BenchDuffs |          8 |   2.627 ns | 0.0227 ns | 0.0213 ns |
// | BenchBlockCopy |          8 |   2.608 ns | 0.0160 ns | 0.0150 ns |
// |     BenchDuffs |         32 |   2.889 ns | 0.0184 ns | 0.0172 ns |
// | BenchBlockCopy |         32 |   2.394 ns | 0.0132 ns | 0.0123 ns |
// |     BenchDuffs |        128 |   6.083 ns | 0.0261 ns | 0.0231 ns |
// | BenchBlockCopy |        128 |   3.904 ns | 0.0223 ns | 0.0208 ns |
// |     BenchDuffs |       1024 |  33.169 ns | 0.1734 ns | 0.1622 ns |
// | BenchBlockCopy |       1024 |  10.607 ns | 0.0625 ns | 0.0584 ns |
// |     BenchDuffs |       4096 | 113.503 ns | 0.3058 ns | 0.2860 ns |
// | BenchBlockCopy |       4096 |  31.632 ns | 0.0957 ns | 0.0895 ns |
using System.Runtime.CompilerServices;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

public class Program
{
    private int _bufferSize;
    private byte[] _bufferFrom;
    private byte[] _bufferTo;

    public static void Main(string[] args)
    {
        BenchmarkRunner.Run<Program>();
    }

    [Params(8, 32, 128, 1024, 4096)]
    public int BufferSize
    {
        get => _bufferSize;
        set
        {
            _bufferSize = value;
            _bufferFrom = new byte[value];
            _bufferTo = new byte[value];
        }
    }


    [Benchmark]
    public unsafe void BenchDuffs()
    {
        fixed (byte* from = _bufferFrom)
        fixed (byte* to = _bufferTo)
            DuffsDevice(to, from, (uint)BufferSize);
    }


    [Benchmark]
    public unsafe void BenchBlockCopy()
    {
        fixed (byte* from = _bufferFrom)
        fixed (byte* to = _bufferTo)
            Unsafe.CopyBlock(to, from, (uint)BufferSize);
    }

    static unsafe void DuffsDevice(byte* to, byte* from, uint count)
    {
        uint r = count & 7;
        from += r;
        to += r;
        switch (r)
        {
            case 7: to[-6] = from[-6]; goto case 6;
            case 6: to[-5] = from[-5]; goto case 5;
            case 5: to[-4] = from[-4]; goto case 4;
            case 4: to[-3] = from[-3]; goto case 3;
            case 3: to[-2] = from[-2]; goto case 2;
            case 2: to[-1] = from[-1]; goto case 1;
            case 1: to[0] = from[0]; break;
            case 0: break;
        }
        ulong* f = (ulong*)from;
        ulong* t = (ulong*)to;
        uint n = count / 8;
        while (n > 0)
        {
            t[n] = f[n];
            --n;
        }
    }
}